Hitherto, there have been known a vertical gradient freezing (VGF) method and a vertical Bridgman method as methods for growing single crystals of compound semiconductors. Further, there have been filed applications concerning methods for controlling various growth requirements for growing good-quality single crystals which have few dislocations or the like.
For example, regarding VGF method, there has been well-known the invention described in the Japanese Patent Application Publication (Examined) No. Tokuko-Hei 5-59873. The gist of this invention is as follows. That is, a single crystal is grown downwardly from the center of the surface of a raw melt by cooling the inside of a heating furnace at a solid-liquid interface temperature gradient of 0.1 to 10.degree. C./cm and at a cooling rate of 0.1 to 1.degree. C./hr while the distribution of temperature in the heating furnace is maintained in such a manner that the center of the surface of the raw melt is at the lowest temperature and that as a temperature measuring position therein becomes radially outwardly and downwardly farther away therefrom, the temperature increases.
Further, regarding Bridgman method, there have been filed applications concerning a method for regulating a furnace wall temperature gradient along the crystal and a furnace wall temperature gradient along the melt on the basis of the coefficients of thermal conductivity of the crystal and the melt, respectively (as described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 2-239181 and a method for detecting temperatures at a plurality of points on the outer circumference of a crucible and for calculating and estimating the temperature distribution in the crucible and the position and shape of the growth boundary face at regular time intervals according to the detected temperatures, thereby controlling the growth requirements (as described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 1-212291).
Moreover, there have been well-known methods of the inventions disclosed in the Japanese Patent Application Publication (Laid-Open) Nos. Tokukai-Hei 2-167882 and 3-183682 other than the previously filed aforementioned applications as a method for growing a crystal by establishing a vertical temperature gradient by use of a vertical type heating furnace similarly as in the cases of VGF method and Bridgman method. In the case of the invention described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 2-167882, a vertical type vessel, on the top of which a nearly inverse-conical portion having a small top opening is provided, is used. Further, the vessel is filled with a raw melt. Moreover, a seed crystal is made to be in contact with the raw melt through the small top opening. Thereby, the solidification of the raw melt is performed downwardly. The invention described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 3-183682 is an improvement of the invention described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 2-167882. In the case of the invention described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 3-183682, a small bottom opening for making excessive melt flow out therefrom is provided in the lower portion of a vertical vessel similar to that used in the method of the invention described in the Japanese Patent Application Publication (Laid-Open) No. Tokukai-Hei 2-167882. Thus, the failure or the like of the vessel due to the expansion of the volume thereof associated with the solidification of the melt can be prevented by making the raw melt flow out from the small bottom opening.
Furthermore, regarding a method of using a lateral type heating furnace like a horizontal Bridgman method and a horizontal gradient freezing method, there has been well-known a method of the invention described in the Japanese Patent Application Publication (Examined) No. Tokuko-Sho 53-5867. In the case of this method, a container containing a melt and a seed (i.e., a seed crystal) is charged into the lateral type furnace. Further, the solidification of the melt is started from the free surface (i.e., the top surface) thereof and is finished on the bottom surface thereof by keeping the temperature on the surface of the melt, which is in contact with the bottom of the container, higher the temperature on the top surface of the melt.
However, although a single crystal, the quality of which is higher than as obtained by the vertical Bridgman method, can be obtained in accordance with the control method described in the Japanese Patent Application Publication (Examined) No. Tokuko-Hei 5-59873, all of the grown crystals (ingots) are not always single crystals. Some of the grown crystals are poly-crystallized. Thus, this method has a problem in that the yield is not so high.
Additionally, in recent years, a good-quality and large-diameter single crystal, which has few crystal defects such as dislocations and is, for instance, 4 inches in diameter, is desired. However, a method, by which such a single crystal is produced with a good yield, has not been established yet.
The present invention has been accomplished in view of such circumstances. An object of the present invention is to provide a method for growing a single crystal, by which good-quality and large-diameter single crystals can be produced with a good yield.